1. Field of the Invention
The invention relates to systems configured to use both timed delay integration and pulsed illumination while owing high-speed image scanning.
2. Related Art
Time delay integration (TDI) is an imaging process that produces a continuous image of a moving object that can be much larger than the field of view of the imaging hardware. In a TDI system, image photons are converted to photocharges in a sensor comprising an array of pixels. As the object is moved, the photocharges are shifted from pixel to pixel down the sensor, parallel to the axis of movement. By synchronizing the photocharge shift rate with the velocity of the object, the TDI can integrate signal intensity at a fixed position on the moving object to generate the image. The total integration time can be regulated by changing the speed of the image motion and providing more/less pixels in the direction of the movement. In conventional TDI inspection systems, the readout circuits are positioned on one side of the sensor to read out the integrated signal. TDI inspection systems can be used for inspecting wafers, masks, and/or reticles.
In a system with continuous illumination and a moving object, the TDI must be precisely synchronized to the image motion so that the recorded image is not blurred. One disadvantage of this system is that the readout of the sensor can be in only one direction, i.e. in the direction corresponding to the image motion, and must operate at the same scan rate as the object during the illumination pulse. In a system with pulsed illumination and a moving object, the image can be collected almost instantly over the entire sensor area. The image can then be read out along both sides of the sensor, thereby effectively doubling the readout speed. The readout line rate can also be faster than the image scan rate without compromising the final image quality, which can further increase readout speed. A critical disadvantage of this system is that the illumination pulse must be very short so that the moving image does not produce blur during the exposure time. As the pulsed illumination time approaches the sensor line period, the image motion will start to cause significant blur, and the image will degrade severely beyond that threshold. Another disadvantage of this system using very short pulses is that the image information at defective pixel locations on the sensor cannot be recovered.
Therefore, a need arises for a method and apparatus that provides a continuously moving object, pulsed illumination, fast readout capability, and recovery of image information where sensor pixels are defective.